When film or negative is developed, the tiny crystals of Silver Halide salts are turned into tiny filaments of metallic silver. These filaments are grouped together in random patterns and form grains. The size of the crystals of silver halide salts is different for different speed film stocks. Normally, the higher the speed the greater the size of the crystal is. For example, ISO 200 rated film is twice as sensitive as a film rated ISO 100. Other film ratings include ISO400, IS0800, IS01600 and IS03200, etc. Normally, film speed is inversely related to granularity, the size of the grains of silver halides in the emulsion. A fine-grain stock is slow, meaning that the amount of light used to expose it must be high. Fast films used for shooting in poor light produce a grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. Graininess is an accumulation of these areas of all light or all shadow that break up the visible contours of the image. An area of the image will consist of bright areas and dark areas with few transitional areas of mid tones. In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of mid tones. In advertising, music videos, and some drama, it is fashionable to create mismatches of grain, color cast, and so forth between shots. These mismatches of grain are deliberate and sometimes are added in post-production.
Current techniques for simulating film grains are directed to adding film grains for still pictures. These techniques are not flexible and not automatic. Further, current film grain simulating techniques do not provide a good mechanism to remove film grain and add back controllable film grain automatically. A known film grain management technique for hard drive-digital video disc (HD-DVD) creates a lookup table for film grain that can be easily applied to audio-video coding, i.e., AVC codec. In hard drive digital video disc applications, the encoder removes the film grains as well as some picture details depending on the bit rate. This film grain management technique does not precisely calculate the grain level and does not automatically characterize the film grain. A film processing technique that can determine film grain patterns or levels automatically is needed.